This invention is generally directed to liquid developer compositions, and more specifically, the present invention relates to the preparation of liquid developers with high solids content wherein supercritical mediums, such as carbon dioxide, are selected. In embodiments, the process of the present invention utilizes attritors and supercritical carbon dioxide, and wherein subsequent to high solids attrition the solids and the medium, such as hydrocarbon fluid, of the liquid developer are removed, transferred to a suitable vessel, such as a known pressure vessel, which vessel is thereafter sealed, and subsequently, there is introduced into the vessel pressured carbon dioxide. There is enabled with the processes of the present invention in embodiments high solids liquid developers, wherein high refers to, for example, from about 20 to about 50 percent of thermoplastic resin, pigment, and charge control agent, or charge adjuvant.
Liquid developers are known, reference for example U.S. Pat. No. 5,308,731, the disclosure of which is totally incorporated herein by reference, wherein there is illustrated a liquid developer comprised of a nonpolar liquid, thermoplastic resin particles, a nonpolar liquid soluble ionic or zwitterionic charge director, and a charge adjuvant comprised of an aluminum hydroxycarboxylic acid, or mixtures thereof; U.S. Pat. No. 5,306,591, the disclosure of which is totally incorporated herein by reference, which discloses a liquid developer comprised of thermoplastic resin particles, a charge director, and a charge adjuvant comprised of an imine bisquinone; and United States Statutory Invention Registration No. H1483, the disclosure of which is totally incorporated herein by reference, which discloses a liquid developer comprised of thermoplastic resin particles, and a charge director, especially the methyl HBR salt thereof, comprised of an ammonium AB diblock copolymer. Moreover, in U.S. Patent No. 5,525,449, the disclosure of which is totally incorporated herein by reference, and wherein there is illustrated a liquid electrostatographic developer comprised of (A) a nonpolar liquid having a Kauri-butanol value of from about 5 to about 30, and present in a major amount of from about 50 percent to about 95 weight percent; (B) thermoplastic resin particles having an average volume particle diameter of from about 5 to about 30 microns, and pigment; (C) a charge director, such as an aliphatic alcohol mixture; and (D) a charge adjuvant. Illustrated in U.S. Pat. No. 5,525,448, the disclosure of which is totally incorporated herein by reference, is a positively charged liquid developer comprised of thermoplastic resin particles, pigment, a charge director, and an insoluble charge adjuvant comprised of a polymeric quaternary ammonium compound, and wherein the charge adjuvant is associated with or combined with said resin and said pigment; and U.S. Ser. No. 204,016, abandoned the disclosure of which is totally incorporated herein by reference, is a liquid developer comprised of thermoplastic resin particles, optional pigment, and a charge director comprised of a mixture of an organic anionic complex phosphate ester and organic aluminum complex, or mixtures thereof of the formulas ##STR1## wherein R.sub.1 is selected from the group consisting of hydrogen and alkyl, and n represents a number.
In the aforementioned U.S. Pat. No. 5,525,449 it is indicated that liquid electrostatic developers can be prepared by a variety of known processes such as, for example, mixing in a nonpolar liquid the thermoplastic resin and colorant in a manner that the resulting mixture contains, for example, about 15 to about 30 percent by weight of solids; heating the mixture to a temperature of from about 70.degree. C. to about 130.degree. C. until a uniform dispersion is formed; adding an additional amount of nonpolar liquid sufficient to decrease the total solids concentration of the developer to about 10 to about 20 percent by weight; cooling the dispersion to about 10.degree. C. to about 50.degree. C.; adding the charge adjuvant compound to the dispersion; diluting the dispersion; and thereafter adding a charge director. In the initial mixture, the resin, colorant and optional charge adjuvant may be added separately to an appropriate vessel such as, for example, an attritor, heated ball mill, heated vibratory mill, such as a Sweco Mill manufactured by Sweco Company, Los Angeles, Calif., equipped with particulate media for dispersing and grinding, a Ross double planetary mixer (manufactured by Charles Ross and Son, Hauppauge, NY), or a two roll heated mill, which requires no particulate media. Useful particulate media include particulate materials like a spherical cylinder selected from the group consisting of stainless steel, carbon steel, alumina, ceramic, zirconia, silica and sillimanite. Carbon steel particulate media are particularly useful when colorants other than black are used. A typical diameter range for the particulate media is in the range of 0.04 to 0.5 inch, or about 1.0 to about 13 millimeters. Sufficient, nonpolar liquid is added to provide a dispersion of from about 15 to about 50 percent solids. This mixture is subjected to elevated temperatures during the initial mixing procedure to plasticize and soften the resin. The mixture is sufficiently heated to provide a uniform dispersion of all solid materials, that is colorant, adjuvant and resin. However, the temperature at which this step is undertaken should not be so high as to degrade the nonpolar liquid or decompose the resin or colorant when present. Accordingly, the mixture is heated to a temperature of from about 70.degree. C. to about 130.degree. C., and preferably from about 75.degree. C. to about 110.degree. C. The mixture may be ground in a heated ball mill or heated attritor at this temperature for about 15 minutes to about 5 hours, and preferably about 1 to about 3 hours. After grinding at the above temperatures, an additional amount of nonpolar liquid may be added to the dispersion. The dispersion is then cooled to about 10.degree. C. to about 50.degree. C., and preferably to about 15.degree. C. to about 30.degree. C., while mixing is continued until the resin admixture solidifies or hardens. Upon cooling, the resin admixture precipitates out of the dispersant liquid. Cooling is accomplished by methods such as the use of a cooling fluid, such as water, ethylene glycol, and the like, in a jacket surrounding the mixing vessel. Cooling may be accomplished, for example, in the same vessel, such as the attritor, while simultaneously grinding with particulate media to prevent the formation of a gel or solid mass without stirring to form a gel or solid mass, followed by shredding the gel or solid mass, and grinding by means of particulate media, or with stirring to form a viscous mixture, and grinding by means of particulate media. The resin precipitate is cold ground for about 1 to about 36 hours, and preferably 2 to 6 hours. Additional liquid may be added at any step during the preparation of the liquid developer to facilitate grinding or to dilute the developer to the appropriate percent solids needed for developing.
The disadvantages of utilizing solvents, introducing undesirable fine metallic particles in the liquid developer which can abrade and destroy components in the liquid developer machine are avoided or minimized with the processes of the present invention. In embodiments, the present invention provides a process for obtaining liquid developers with a high solids contents and a desirable size without requiring a concentration step, such as pressure filtration or centrifugation, following attrition, and the process avoids the use of hydrocarbon dilution of the toner during manufacture and the need to recover and recycle or dispose of the hydrocarbon.
Also, reference is made to the following copending patent applications and patents, the disclosures of which are totally incorporated herein by reference, U.S. Pat. No. 5,514,513; U.S. Pat. No.. 5,514,514 and U.S. Pat. No. 5,516,618.which illustrate supercritical processes for the preparation of carrier powder coatings.